This invention relates to the area of polymerization and more particularly to the polymerization of unsaturated hydrocarbons. Polymerization generally consists in reacting together compounds that comprise an unsaturation or a reactive function: monomers. The products that are obtained at the end of the reaction are in the form of long chains in which the same pattern is repeated: these are the polymers. Very often, the polymerization effluent also contains monomers that have not reacted; at least a portion of these monomers are generally recycled toward the polymerization zone, and the device is then designated as a "polymerization loop."
Polymerization can be assisted by comonomers. These comonomers, if they enhance the reactivity of the monomers and the quality of the polymers that are produced, are less reactive than the monomers, however. These are generally expensive compounds; it is therefore advantageous to provide for the recycling of comonomers that have not reacted.
In the process according to the invention, the comonomer that is used is an alpha-olefin. When a comonomer is introduced into the polymerization zone, isomers of this comonomer are also very often introduced; moreover, during polymerization, the comonomer is also unavoidably formed from isomers of the comonomer (internal olefins).
These isomers are not reactive; in general they are inert relative to the polymerization reaction. If no provision is made for purging of the polymerization loop, these isomers accumulate and can then represent a large portion of the fluid that is assumed to contain basically the comonomer that circulates in the polymerization loop. The isomers then play a role of contaminating the polymerization loop.
One skilled in the art knows that it is often very difficult to separate the internal olefins from the alpha-olefins, whereby the internal olefins have in particular boiling points that are close to that of the corresponding alpha-olefin. Consequently, provision is generally made for significant purging. It has the drawback of the loss of comonomers, however.